The present invention relates to a method for producing diffused, contacted and surface passivated semiconductor wafers for semiconductor devices. Semiconductor wafers in this stage of production, which hereinafter will be briefly called "unit cells", are thereafter installed, if and when required, in a housing, a thick film circuit or a printed circuit.
According to known manufacturing processes, semiconductor devices, such as diodes, transistors, thyristors or triacs, for example, are produced by changing the electrical characteristics of the surface of homogeneously doped semiconductor wafers, either wholly or in part, by the introduction of further impurity forming elements, using alloying and/or diffusion processes, to a given depth so that an arrangement of layers and regions of different conductance and different conductivity types is created.
For economic reasons it is advisable not to produce small devices individually but rather to perform the doping and metallization on relatively large wafers which are then divided into individual devices, for example, by etching, sawing, sandblasting or scoring and breaking. These individual devices are finally connected with the current conducting leads by pressure, solder, welded or glued contacts, in the usual manner. In order to protect the semiconductor surfaces against damage and the influence of the ambient atmosphere, the semiconductor wafers are then installed in hermetically sealed housings which, if required, are filled with an inert gas, or are cast or pressed into an insulating material, for example, epoxy resin or silicone resin.
It is a characteristic feature of these known processes in which a plurality of small individual elements are produced from one large silicon wafer, that for complicated structures the metal contact layer must be applied via a masking step. This contact layer must resist the attacks of the separating and etching steps, which limits the selection of contacting metals that can be used, often to the detriment of the subsequent connecting steps.
Devices produced in this manner are distinguished by high sensitivity which requires an uninterrupted flow of process steps including, in particular, the subsequent encasing in a housing or embedding in a protective insulating material. This constitutes a drawback since the separation into quality classes and elimination of rejects can usually be effected only with finished devices which are installed in a final housing.
Semifinished devices are known, such as glass passivated thyristors and triacs which are provided with solder, for example, or devices which are produced according to the planar technique. For these structural types the yield of devices which block at about 1000 V and which are thus particularly desired in a production process directed toward highly blocking devices is poor. Moreover, these devices cannot be fully measured and tested, for example, they cannot be tested under high currents, because no suitable contacts have as yet been applied. Under such test conditions it is impossible to make a final determination of all the characteristics or parameters of the device which are to be expected particularly concerning the current carrying capability, and which will result only after insertion of the device in a housing. It has been found that in the presently employed processes the characteristics determined after installation in a housing often differ from the desired values and from the values previously measured on the individual devices prior to installation.
Aside from the fact that the known embodiments cannot be fully measured and tested before installation in a housing in order to determine a lack of desired characteristics, there hardly exists a possibility, after installation in a housing, to then eliminate a noted lack of sufficient parameters by subsequent correction, for example, by renewed etching of the semiconductor wafers. Often the gold layer which is advisable for good solderability and etchability substantially worsens the properties of the soft solders used for the connections. Since there is thus practically no favorable way to subsequently improve the characteristics of the devices, the proportion of inferior devices and rejects in the total production is correspondingly relatively high in the known processes.
A further drawback of the known processes is that storage, which for the above reasons will usually be limited only to the finished devices, is rather expensive. Moreover, the storage facility is poorly adaptable to a changing requirement for devices of different sizes unless the adaptation is achieved exclusively by a mere increase in the size of the storage facility which again would be uneconomical.
Finally, the high sensitivity of the known embodiments of semiconductor wafers must be considered to be a disadvantage. Since the semiconductor body, for example, the silicon body, is not or only insufficiently protected against mechanical damages, the transport of the devices presents a problem. Thus the selection of a more favorable location for further processing is either limited or impossible. Moreover, the devices are poorly suited for process or testing steps in a production sequence which involves highly mechanized or automatic production methods.